Physical oceanography is more and more being asked to provide descriptions and forecasts of ocean environmental parameters for climate, fisheries and ice forecasting purposes. XBT's (Expendable BathyThermograph), surface drifters and satellite SST (Sea Surface Temperature) sensors have provided the tools to measure the upper ocean temperature, but in many areas of the ocean the salinity is also important in determining mixed layer properties and dynamic height for velocity calculations. Over most of the world's oceans, the seasonal layer of the ocean is less than 300 m deep, but in the high northern latitudes were salinity is particularly important, winter mixed layers can exceed 1000 m in depth. Salinity measurements can be obtained using towed profiling instrumentation, but these systems require the use of hard faired cables in order to achieve operating range of 300 to 500 m. This makes these systems complex, expensive and difficult to deploy and recover.
Various devices are known or have been proposed for the collection of data relating ocean properties such as temperature and salinity with respect to depth. Most of these devices involve deploying a probe or "fish" from a vessel and transmitting the data to the vessel as the probe descends. These are commonly known in the art as a CID (Conductivity-Temperature-Depth). Examples of prior devices are disclosed in U.S. Pat. No. 3,339,407 to Campbell, et al, and U.S. Pat. No. 3,397,573 to Carter. In these patents the probes described are expendable.
Some expendable instruments are able to achieve deep profiles from vessels underway because the line connecting the probe with the vessel pays out from both the probe and the launcher. Thus the line does not move relative to the water and there is no drag force. Applying this principle for a recoverable body may be theoretically possible, but rewinding the line into the probe following each deployment would be a very complex operation.
In order to facilitate economical acquisition of data, and also to avoid lo discarding probes on the bottom of the ocean, it would be desirable to be able to provide automated repeating deployment and retrieval of a descending probe while the vessel is in motion. Preferably, the system should allow continuous operation with various types of vessels in transit at the normal velocity of the vessel. Further, it would be desirable to be able to control operation with little human intervention, for example, from the bridge of the vessel.
Achieving these goals is made increasingly difficult at higher vessel velocities. Higher vessel velocities means that longer lines are required to achieve a given probe depth. In turn, longer lines result in higher drag which slows the descent rate, and increases recovery load. Hence, higher velocities tend to limit o the depth that can be practically achieved.